Submit many renders at once (e.g. a product catalog)

Body: {items: [{ref, scene_url | scene_id, outcome?, job?, notify_url?}], outcome?: turntable|product-shot, defaults?: {...}, notify_url?}. Up to 100 items. Returns 202 with batch_id; each item is fetched and submitted in the background (same quota and limits as single jobs; an item that fails gets its own error and the rest continue). Idempotency-Key supported.